LibGUI: Start adding an automatic widget layout system.

My needs are really quite simple, so I'm just going to add what I need
as I go along. The first thing I needed was a simple box layout with
widgets being able to say whether they prefer fixed or fill for both
their vertical and horizontal sizes.

I also made a simple GStatusBar so FileManager can show how many bytes
worth of files are in the current directory.

SharedGraphics/Size.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <AK/AKString.h>
+
 struct GUI_Size;
 
 class Size {
@@ -29,8 +31,17 @@ public:
         return !(*this == other);
     }
 
+    Size& operator-=(const Size& other)
+    {
+        m_width -= other.m_width;
+        m_height -= other.m_height;
+        return *this;
+    }
+
     operator GUI_Size() const;
 
+    String to_string() const { return String::format("[%d,%d]", m_width, m_height); }
+
 private:
     int m_width { 0 };
     int m_height { 0 };